Multi-layer articles can be used in a variety of applications. Biaxially textured metal oxide buffer layers on metal substrates are potentially useful in electrical and/or electronic devices where an electrically active layer is deposited on the buffer layer. The electrically active layer may be a superconductor, a semiconductor, or a ferroelectric material. Typically, such articles 10 include a metal substrate 11, buffer layer(s) 12, and an active layer 13, e.g., a superconductor, as illustrated in FIG. 1. The metal substrate, such as Ni, Ag, or Ni alloys, provides flexibility for the article and can be fabricated over long lengths and large areas. Metal oxide layers, such as LaAlO3, CeO2, or yttria-stabilized zirconia (YSZ), make up the next layer and serve as chemical barriers between the metal substrate and the active layer. The buffer layer(s) can be more resistant to oxidation than the substrate and can reduce the diffusion of chemical species between the substrate and the superconductor layer. Moreover, the buffer layer(s) can have a coefficient of thermal expansion that is well matched with the superconductor material.
To achieve high critical current densities in the article, the superconducting material is biaxially textured and strongly linked. As used herein, “biaxially textured” refers to a surface for which the crystal grains are in close alignment with a direction in the plane of the surface and a direction perpendicular to the surface. One type of biaxially textured surface is a cube textured surface, in which the crystal grains are also in close alignment with a direction perpendicular to the surface. Examples of cube textured surfaces include the (100)[001] and (100)[011] surfaces, and an example of a biaxially textured surface is the (113)[211] surface.
Typically, the buffer layer is an epitaxial layer, that is, its crystallographic orientation is directly related to the crystallographic orientation of the surface onto which the buffer layer is deposited. For example, in a multi-layer superconductor having a substrate, an epitaxial buffer layer and an epitaxial layer of superconductor material, the crystallographic orientation of the surface of the buffer layer is directly related to the crystallographic orientation of the surface of the substrate, and the crystallographic orientation of the layer of superconductor material is directly related to the crystallographic orientation of the surface of the buffer layer. Therefore, the superconducting properties exhibited by a multi-layer superconductor having a buffer layer can depend upon the crystallographic orientation of the buffer layer surface.
The conventional processes used to grow buffer layers on metal substrates and achieve this transfer of texture include vacuum processes such as pulsed laser deposition, sputtering, and electron beam evaporation. Such techniques are disclosed in, for example, A. Goyal, et al., “Materials Research Society Spring Meeting, San Francisco, Calif., 1996; X. D. Wu, et al., Appl. Phys. Lett. 67:2397, 1995). Solution phase techniques, including metalorganic deposition (MOD), are also used to grow buffer layers. Such techniques are disclosed in, for example, S. S. Shoup et al., J. Am. Cer. Soc., Vol. 81, 3019; D. Beach et al., Mat. Res. Soc. Symp. Proc., vol. 495, 263 (1988); M. Paranthaman et al., Superconductor Sci. Tech., vol. 12, 319 (1999); D. J. Lee et al., Japanese J. Appl. Phys., vol. 38, L178 (1999) and M. W. Rupich et al., I.E.E.E. Trans. on Appl. Supercon. vol. 9, 1527 (1999).
When using solution phase techniques to deposit epitaxial oxide films on oxidizable metal or metal alloy substrates, it is necessary to carry out the process in a reducing atmosphere relative to the oxidation potential of the metal or metal alloy of the substrate. This is illustrated in FIG. 3, which shows the metal/metal oxide phase stability line for Ni and W as a function of temperature and oxygen pressure. Typically for substrates such as nickel or NiW alloys, a hydrogen/argon environment is employed, e.g., 4%/96% H/Ar. See, U.S. Pat. No. 6,077,344. The use of the reducing environment typically requires higher temperatures, e.g., of about 1000–1200° C., to completely decompose the organic components of the precursor film and to nucleate and crystallize the oxide phase. Although these conditions produce oxide films with epitaxial texture, the high temperatures frequently result in undesirable grain growth and roughening of the oxide film surface. The surface roughness increases as the thickness of the oxide layer increases. Also the high processing temperatures increase the rate of metal diffusion through the buffer layer causing potential contamination of the buffer and subsequent layers.
Under oxidizing conditions, it may be possible to directly deposit a textured oxide buffer layer on a metal substrate, particularly if the metal is not overly sensitive to oxidation, e.g., nickel. See, for example, U.S. Pat. No. 6,440,211. Direct growth of epitaxial buffer layers on a metal substrate under even mildly oxidizing conditions has not been successful to date, however, because the metal surface inevitably forms a metal oxide layer which may have a crystal structure different than the underlying metal and which disrupts epitaxial growth.